This is a very loose adaptation of a recipe from Cook's Illustrated magazine. This is really, really good; The biggest problem is these margaritas do go down very easily.
Adapted from the July-August 2000 Issue of Cook's Illustrated Magazine
4 limes, juiced
2 lemons, juiced
1 small orange, juiced
1 cup Reposado tequila (I use Hornitos)
1 cup Triple Sec (I use Bols and never substitute Cointreau)
Glug of Rose's Lime Juice (optional)
Salt to rim the glass (optional)
Mix all of the above in a pitcher. Add simple syrup* to taste; the amount you add will depend on how tart the fruit is and on how sweet you like your drink. Remember, the Triple Sec is sweet, and the Rose's Lime has sugar in it too even though it imparts that sweet/sour taste.
At this point, taste a little of the mixture to make sure it's sweet enough - you can always add more simple syrup until you get it right, but you don't want it too sweet. After all, this is a drink for grown-ups.
In batches (because it won't all fit), shake the above mixture from the pitcher in a metal cocktail shaker filled with ice until the shaker is frosty. Serve over ice cubes in old-fashioned glasses or large wine glasses.
Personally, I do not rim the glasses with salt and never use margarita glasses, but if you must, feel free.
Bring equal amounts of sugar and water to a boil, just until the sugar dissolves. This will keep in a jar in the refrigerator. It's good to have on hand in the summer for iced tea, lemonade, and refreshing cocktails.